[tor-bugs] #26972 [Core Tor/Tor]: Create CI task to ensure that all Rust files have been formatted with rustfmt

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#26972: Create CI task to ensure that all Rust files have been formatted with
rustfmt
--------------------------+------------------------------------
 Reporter:  chelseakomlo  |          Owner:  teor
     Type:  enhancement   |         Status:  needs_revision
 Priority:  Medium        |      Milestone:  Tor: 0.3.5.x-final
Component:  Core Tor/Tor  |        Version:
 Severity:  Normal        |     Resolution:
 Keywords:  rust          |  Actual Points:
Parent ID:                |         Points:
 Reviewer:  teor          |        Sponsor:
--------------------------+------------------------------------
Changes (by teor):

 * status:  merge_ready => needs_revision


Comment:

 I'd like to merge this branch after #24629 merges.

 I'm happy to do the merge, it only affects 5d749e4bde:
 * install rustfmt in .travis.yml after the rewrite

 We should also:
 * print the rustfmt version, like we do for rustup in my 14ceee0fd1 in
 https://github.com/teor2345/tor.git

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26972#comment:15>
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